1. Field of the Invention
The present invention relates to an optical information recording medium for recording and reproducing information with high density by using a laser beam.
2. Description of the Related Art
In order to write a large volume of information into an optical information recording medium, a laser beam having a small diameter is used for heating selected microscopic areas of the recording medium. The optical characteristics of the heated areas of the recording layer of the recording medium is changed, and the change thereof can be optically read out by using a laser beam for reproducing.
A laser beam having a size in the order of a wavelength of the laser beam can be obtained by focusing a laser beam emitted from a laser diode with a lens system. Such a small laser beam can provide a large amount of energy to a unit area of an optical recording medium, even when a laser diode with a low power output is used as a light source.
A basic structure of a conventional optical information recording medium includes a recording thin film layer which is optically changed by the irradiation of a laser beam formed on a flat substrate.
The procedures of recording and reproducing information are conducted as follows: PG,3
While a flat-plate shaped recording medium is moved by a rotating means or a translating means such as a motor, a laser beam is focused onto a recording thin film of the recording medium. The energy of the laser beam is absorbed into the recording thin film, resulting in the rise in temperature of the recording thin film.
In a writing mode, the recording thin film is irradiated with a laser beam whose optical output power level is varied across a certain threshold power in response to modulation signals. The optical state of the irradiated portion of the recording thin film is changed, whereby information is recorded therein. The threshold power depends on thermal characteristics of the substrate, a relative velocity to the laser spot of the recording medium and the like, as well as characteristics of the recording thin film themselves.
In a reproducing mode, the recorded part and non-recorded part of the recording thin film are irradiated with a laser beam whose output level is sufficiently lower than the threshold power. The changes in the optical characteristics of the recorded part and the non-recorded part are optically detected, whereby information recorded in the recording medium is reproduced. Herein, the recorded part corresponds to a portion where information is recorded; the non-recorded part corresponds to a portion where no information is recorded.
A structural phase changeable type recording medium includes a recording thin film whose optical constant is changeable without almost any change in shape. An optical constant of the recording thin film can be changed by changing crystallinity of the recording thin film. In more detail, the state of the recording thin film is changed between an amorphous state and a crystalline state by the irradiation of the laser beam, and at least either the extinction coefficient or refractive index of the recording thin film is changed, whereby information is recorded.
Between the both states of the recording thin film, the reflectance, i.e., the amplitude of reflected light, is varied, so that the change in the intensity of the reflected light is detected by a detection system, whereby information is reproduced. Since the structural phase changeable type recording medium is not changed in shape, an air-sandwich structure such as an ablative type medium is not required. Therefore, the recording medium can be fabricated in a simple way with a low cost, requiring a simple adhesion protect structure or the like.
As a recording thin film material, an amorphous chalcogenide thin film can be used. Examples thereof include an oxide thin film mainly containing tellurium (Te) and tellurium oxide (TeO.sub.2), a thin film mainly containing tellurium (Te), tellurium oxide (TeO.sub.2) and Palladium (Pd), and a thin film mainly containing Germanium (Ge), Antimony (Sb) and tellurium (Te). These thin films are disclosed in Japanese Patent Publication No. 54-3725 and U.S. Pat. No. 3,971,874; Japanese Patent Publication No. 2-9955 and U.S. Pat. No. 4,624,914; and Japanese Laid-Open Patent Publication No. 62-209742, Japanese Laid-Open Patent Publication No. 63-225934 and "High speed overwritable phase change optical disk material" (Japanese Journal of Applied Physics, vol. 26 (1987) suppl. 26-4), respectively.
In the structural phase changeable type optical recording medium, an "overwrite" in which new information can be recorded while erasing information already recorded can be carried out by modulating the power level of a laser beam between a relative high recording power and a relative low erasing power (see Japanese Patent Publication No. 2-58590). An amorphous state is formed by heating the thin film material over the melting point and quenching it by the recording power. Further, a crystalline state is obtained by heating the thin film material up to the temperature for crystallization by a relative low erasing power. Portions irradiated with the laser beam of the recording power become amorphous, and portions irradiated with that of the erasing power become crystalline. Accordingly, even though the recording thin film material is in any type of optical state before recording, the overwrite record can be realized by radiating the laser beam only one time.
The structural phase changeable type optical recording medium includes a substrate, a first transparent layer having a refractive index different from that of the substrate, a recording thin film layer, a second transparent layer, and a reflective layer. An inorganic dielectric having a high melting point is used for the transparent layer, and metal is used for the reflective layer. Due to such a structure, when the recording thin film layer is subjected to the structural phase change, i.e., especially when the state of the layer is changed into an amorphous state, the shape thereof is hardly changed even after being put in the melted state. In addition, the structural phase changeable type optical recording medium is usually formed by selecting the thickness of respective layers so as to increase the reflectance change in structural phase change and optical absorbance.
Since the structural phase changeable type optical recording medium can reproduce information by changing the intensity of reflected light, the medium can be compatible with a recording medium (replica disk) in which information previously recorded by uneven pits, such as a video disk (brand name: laser disk) and a digital audio disk (brand name: compact disk), under the condition that the absolute value of the reflectance of such mediums is large. That is, signals can be reproduced in such a player that only has a function of reproducing signals. The replica disk usually includes a metal thin film having a large reflectance made of aluminum (Al) or the like, provided on a resin substrate on which uneven pits are formed, so that a flat portion thereof has 70% or more of reflectance. In general, as the reflectance comes close to this value, the optical recording medium is likely to be compatible with the replica disk.
However, in a conventional structural phase changeable type optical recording medium, a laser beam is hardly transmitted through the medium; instead, the laser beam is partially absorbed in the medium, and the remaining beam is reflected. The laser beam may be absorbed in the reflective layer, whose absorption can be neglected.
Further, the optical recording medium reproduces signals by detecting the reflectance change; however, when the modulation obtained by the ratio of the reflectance change after recording to the reflectance before recording is increased, the reflectance of either the amorphous state or the crystalline state should be increased and that of the other decreased. In this case, the optical absorbance of each state will be reverse with respect to the case of the reflectance. Each reflectance of the non-recorded state (crystalline state) and the recorded state (amorphous state) is defined, for example, as Rc and Ra. Further, each optical absorbance is defined as (1-Rc) and (1-Ra), the ratio thereof being (1-Rc)/(1-Ra ). When new information is overwritten in the recording medium whose ratio (1-Rc)/(1-Ra) is large, the new information cannot be appropriately recorded for the following reason: Although the amorphous portion and the crystalline portion are respectively irradiated with the laser beam of the same power, the energy of the laser beam is not absorbed with the same degree. As a result, a final temperature and a size of an area where the signals are recorded are different in each portion, and the shape of the area is distorted. In the conventional structural phase changeable type recording medium, the reflectance of the laser beam is 20% to 30% in the crystalline state and 0% to 10% in the amorphous state, the difference thereof being approximately 20%. In this case, the absorbance is 70% to 80% and 90% or more, respectively. Therefore, the ratio (1-Rc)/(1-Ra) is comparatively small, resulting in small influence on overwrite characteristics. In order to increase the modulation by making the reflectance close to 70% in the non-recorded state, the reflectance of the recorded state is made, for example, 30% or less, so that the optical absorbance of the crystalline state becomes 30% or less; the optical absorbance of the amorphous state becomes 70% or more, whereby the ratio thereof becomes double or more. In this case, overwriting cannot substantially be conducted.
As described above, the structural phase changeable type recording medium cannot conduct the overwrite nor be compatible with the replica disk, under the condition that the reflectance is large.
A method for improving erase characteristics by adjusting the optical absorbance difference of the recording thin film has been proposed (Japanese Laid-Open Patent Publication No. 1-149238). According to the method, the laser beam can be absorbed into the recording thin film layer almost equally in the amorphous state and the crystalline state, thereby improving the overwrite characteristics. Further, the following is also disclosed: Although the thin film in the amorphous state absorbs the same energy of the laser beam as that in the crystalline state, the amorphous state becomes higher because of the different thermal conditions caused by the thermal material difference, such as thermal conductivity and latent heat of fusion. Therefore, a larger absorbance can be obtained in a more crystalline state. However, also in this case, signals are reproduced by taking advantage of the reflectance change, so that it is difficult to achieve a recording medium with a high reflectance and also a large reflectance change and modulation.